1. Field of the Invention
The present invention relates to the transmission of electrical signals between two devices which are connected by contacts, and more particularly to the transmission of different signals from one of the devices to the other and vice versa.
Even more specifically, the invention relates to the bidirectional transmission between the devices of signals in the form of electrical currents which correspond to logic states which are generated in one of the devices and have to be transmitted to the other.
2. Description of the Prior Art
A system for bidirectional data transmission is described, for example, in co-pending patent application Ser. No. 875,564 now U.S. Pat. No. 4,163,210 entitled "Arrangements for Checking a Contact Inserted Between a Transmitter Circuit and a Receiver Circuit to Allow Electrical Signals to be Transmitted" which was filed on Feb. 6, 1978 in the name of the present applicant and which is assigned to the assignee of the present invention. The subject matter of said application is hereby incorporated by reference. In a system of this kind, it is generally necessary to provide, in addition to a return contact, as many connecting circuits, i.e. contacts, as there are types of electrical signal to be transmitted in one direction or the other.
Thus, when the arrangement described in the above-mentioned patent is used in cases where the transmitter circuit is a data processing system and the receiver circuit is a credit card, the transmitter circuit has to transmit signals of two kinds to the receiver, namely data signals and synchronizing clock signals, and the receiver circuit has to transmit signals representing the readout of data recorded in the credit card to the transmitter. It is, therefore, necessary to provide four connections via four separate contacts.
It is therefore a principal object of the present invention to reduce the danger of malfunction or data transmission anomalies attributable to problems with the contacts, by reducing the number of such contacts and further to provide a bidirectional data transmission technique which, in combination, produce a greatly improved data transmission system.